1. Specific Binding Assays
Methods for measuring immunochemical or other types of specific binding reactions have become widely accepted in the field of medical testing in recent years. Generally speaking, an immunochemical reaction involves the reaction between at least one antigen and at least one antibody. An antigen is ordinarily a substance, such as a protein or carbohydrate, which is capable of inducing an immune response; i.e., antibody production, when introduced into an animal or human body. The antibodies produced as a result of the immune response are bivalent in nature, generally being depicted as a "Y", wherein each arm of the "Y" is capable of binding to the antigen which induced production of the antibody. The presence of a particular antigen or antibody in a patient's test sample may indicate a disease state or a bodily condition, such as pregnancy. An immunochemical reaction is one type of specific binding reaction.
Antibody fragments are often used in addition to or in place of whole antibodies in an immunoassay. Generally, there are three different types of antibody fragments. The first type of fragment is designated as either Fab or F(ab), and is a single arm of the antibody which has been directly cleaved from the whole antibody, usually through digestion by the enzyme papain. Each Fab fragment is monovalent, and has a molecular weight of about 50,000 Daltons, compared to the approximate 150,000 Dalton size of the whole antibody. The second type of fragment is designated as F(ab').sub.2, and consists of both antibody arms, still linked together, but minus the tail which is removed by pepsin digestion. The divalent F(ab').sub.2 fragment has a molecular weight of about 100,000 Daltons, and can be further cleaved into two separate monovalent Fab' fragments (the third type of antibody fragment), also designated as F(ab'), each having a molecular weight of about 50,000 Daltons.
The site on the antigen to which an arm of the antibody binds is referred to as an epitope. Most antigens are polyepitopic, having multiple, and often repeating, binding sites for antibodies. It is the polyepitopic nature of antigens and the bivalent character of antibodies, including F(ab').sub.2 fragments, which enable large antibody:antigen complexes of varying sizes, otherwise known as immunocomplexes, to be formed in an immunoassay.
One particular type of immunoassay which takes advantage of this feature is the sandwich immunoassay, wherein a ternary immunocomplex is formed. The most common type of sandwich immunoassay employs a first insolubilized antibody, usually bound to a solid support, and a second labeled antibody. Each antibody is specific for the antigen of interest (i.e., the analyte to be measured) and binds to a different epitope on the antigen. Preferably, the first antibody binds to an epitope which is remote from the epitope to which the second antibody binds. A ternary complex of insoluble antibody:antigen:labeled antibody is formed where the antigen of interest is contacted with the first and second antibodies. Because each antibody is required to bind to only one antigen, all three types of antibody fragments may be used in this type of method. The presence or absence of the antigen of interest is indicated by the presence or absence of the labeled antibody on the solid support. Ordinarily, the insolubilized phase of the reaction must be separated from the liquid phase in order for either the bound or free labeled antibody to be quantified. Such a reaction is referred to as a heterogenous type of reaction, due to the required separation step.
Nephelometry and turbidimetry require the formation of large aggregates of, e.g., antibody and antigen. Because each antibody must bind to two
different antigen molecules, the monovalent Fab and Fab' fragments are generally ineffective in these methods. The large aggregates cause a change in the light scatter of the solution, and are capable of measurement by nephelometric or turbidimetric methods. These methods do not require the use of traditional labels, such as enzymes, radioactive isotopes, or fluorescent or chemiluminescent compounds, to detect the amount of complex formed. Rather, nephelometric and turbidimetric methods directly measure the amount of complexation. Because no separation step is required, nephelometry and turbidimetry are referred to as homogenous immunoassays.
The multiepitopic nature of the antigen and bivalent character of the antibody will, depending on the amount of antigen and/or antibody present, allow the formation of antigen:antibody complexes large enough to scatter light. Ordinarily, an excess of antibody is used in conjunction with a finite amount of antigen obtained from, e.g., a patient's blood, serum, cerebrospinal fluid (CSF), or urine sample. In such a case, the amount of antigen present in the sample will be the limiting factor in determining the amount and size of antigen:antibody aggregates formed.
In turbidimetry, the reduction of light transmitted through the suspension of particles, or aggregates, is measured. The reduction is caused by reflection, scatter, and absorption of the light by the aggregates. In nephelometry, it is the light scattered or reflected toward a detector that is not in the direct path of light which is measured. In both turbidimetry and nephelometry, the rate of change in light scatter may also be measured as an indication of the amount of antigen present.
Nephelometric procedures have become a convenient method for monitoring antigen:antibody reactions at an early stage, by detecting the rate of growth of complexes capable of scattering light before the complexes separate out of solution as immunoprecipitates. The growth of these complexes begins as a buildup of aggregates which ultimately become large enough to function as "scattering centers". Sternberg, J. C., A Rate Nephelometer for Measuring Specific Proteins by Immunoprecipitation Reactions, Clin. Chem., 23:8, 1456-1464 (1977). The formation of scattering centers can be accelerated by the use of hydrophilic nonionic polymers, such as dextran or polyethylene glycol, which increase the probability of protein-protein interaction by excluding a significant fraction of water. The use of polymers in an immunonephelometric assay also gives the advantages of increased sensitivity and less antiserum consumption.